Ephemeris Accuracy

[marshead]

Hello,

I was trying to create the future near flyby of Toutatis to the Earth on 2008 November 9.51 (0.05025 AU). I could never get the asteroid anywhere near the Earth.

Is Celestia not set up to place these objects accurately (the small bodies) or am I just doing something wrong?

Fantastic software! It could put a few space artists out of business!!!

Marshead

[donkey]

You're right, it's too far, but the closest moment will be in 29th of September, 2004, but this won't work in Celestia eihter.
http://echo.jpl.nasa.gov/asteroids/4179 ... tatis.html
http://www.wilders.force9.co.uk/BeyondEarth/comets.htm

[chris]

The position of the Earth should be correct. I'll check out the orbital elements I have for Toutatis--I may have bad data, or it may be the case that an ellipse isn't a good enough approximation to the actual orbit of Toutatis to predict this event.

--Chris

[hank]

The problem is that in an n-body system, the orbits of objects aren't
actually Keplerian ellipses, although they are approximately so when
viewed over short time intervals.

The Keplerian elements for the approximating elliptical orbit at a specific
point in time are called the osculating elements. Asteroid orbits are
commonly given in this form.

We might be able to get the osculating elements for the epoch of the
encounter from the Minor Planets Center. It would be helpful if Celestia
could support time-dependent orbit specifications.

- Hank

[chris]

The problem is that in an n-body system, the orbits of objects aren't
actually Keplerian ellipses, although they are approximately so when
viewed over short time intervals.

The Keplerian elements for the approximating elliptical orbit at a specific
point in time are called the osculating elements. Asteroid orbits are
commonly given in this form.

We might be able to get the osculating elements for the epoch of the
encounter from the Minor Planets Center. It would be helpful if Celestia
could support time-dependent orbit specifications.

Celestia supports two types of orbits right now--elliptical orbits, and custom orbits coded in C++. The orbits for all the major planets and the Moon are all handled by custom calculations; Celestia 1.2.2 adds custom orbital calculations for the large satellites of Jupiter and Saturn. All the minor planets in solarsys.ssc are just the osculating elements from whatever time I happened to add the object to the file. How are the orbits of minor planets usually specified? All I've ever seen are osculating elements. Is there data for better models of asteroids out there? Precessing ellipses perhaps?

--Chris


--Chris

[marshead]

A gravitational peturbation is the only way you can really change an orbit (excluding things like the precession of Mercury's orbit, or any body that passes really close to the Sun) and while Toutatis is subject to this (the laundry list of close approaches should be an indicator of this), I don't see how a few close passes to the Earth would drive the orbit this far off. There are other small effects that creep up in a true N-body system (like the solar system) but they take a very long time to manifest themselves in orbit errors and peturbations.

I suspect that the elements for Toutatis may not be entirely correct. From the Minor Planet Center website http://cfa-www.harvard.edu/cfa/ps/mpc.html

a = 2.5100537
e = 0.6342274
i = 0.46959
Node = 128.24788
Peri. = 274.78070
Epoch = 2452200.5 (2001 Oct. 18.0)
M = 86.28120

I know that these aren't in the MPC standard format, but I am a Dance of the Planets user from back at the dawn of time (over 10 years now!) and their ordering of the orbital elements has always made the most sense to me so it is habit to list them that way. The information is correct though.

The ephemeris service at the MPC site will let you get the current MPC orbital elements for any object of interest. Just make sure that you click the appropriate radio dial (I find the 8 line elemets to be the most useful).

Looking at the orbit in Celestia, it appears that the biggest problem is not the shape of the orbit, but the placement of the asteroid in the orbit (the mean anomaly, M, and the epoch of observation). An inspection of the orbit used and the MPC orbit may reveal the problem.

I am studying Near-Earth Asteroids as an observer at the University of Western Ontario. Beginning as soon as the weather improves, we will be using the 1.22-m telescope that belongs to the University on a regular basis to determine sizes, shapes and surface properties of newly discovered NEOs. If you have any questions about the orbits, I'll do my best to answer them.

-Marshead[/url]

[chris]

Much better! Marshead, thanks for the orbital elements . . . I replaced the old orbit for Toutatis with this one:

Code: Select all

   EllipticalOrbit
   {
   Period            3.976715
   SemiMajorAxis     2.5100537
   Eccentricity      0.6342274
   Inclination       0.46959
   AscendingNode   128.24788
   ArgOfPericenter 274.78070
   MeanAnomaly      86.28120
   Epoch        2452200.5
   }


The encounter is still not quite right . . . The closest approach is about 3.5 million km instead of 1.5 million, the closest approach date is 2004 Oct 4 instead of 2004 Sep 29. Is this a reasonable amount of error to expect from using an unperturbed ellipse to model the orbit of Toutatis?

--Chris

[Vicware]

I've visited every satellite orbiting planets, and all the comets, and
I've not seen a single body spinning in more than 1 axis. Is this the
way it works, or is it a wish list item?

[chris]

I've visited every satellite orbiting planets, and all the comets, and
I've not seen a single body spinning in more than 1 axis. Is this the
way it works, or is it a wish list item?

It's another wish list item . . . I would like to model more complex rotations in the future. If you check the to do list, I think I mention Hyperion, a moon of Saturn which has a chaotic rotation. The rotation of Toutatis is simpler, due to (I believe) off-axis terms in its moment of inertia tensor. I need to look into the mathematics of such rotations some more--anyone here know how to model them analytically?

--Chris

[Rassilon]

Epoch = 2452200.5 (2001 Oct. 18.0)

-Marshead[/url]

Chris, Does the engine initiate the same epoch of bodies every time it loads the ssc file? Or does this compinsate with the current timeline?

[CAP-Team]

Sorry to kick this old thread, but with these orbital elements I get a closest approach to earth of Toutatis on 9 November 2008 at a distance of 7.5 million kilometers (= 0.05 AU).

Code: Select all

EllipticalOrbit {
  Epoch       2454771.5       # 2008 NOV 01 00:00UT
  Period            4.03269897523155
  SemiMajorAxis     2.53355616628404
  Eccentricity      0.628990346621832
  Inclination       0.44552268775922
  AscendingNode   124.532940115730
  ArgOfPericenter 278.5457039835090
  MeanAnomaly     358.756418826834
}


[BobHegwood]

Sorry to kick this old thread, but with these orbital elements I get a closest approach to earth of Toutatis on 9 November 2008 at a distance of 7.5 million kilometers (= 0.05 AU).

Pardon my intrusion here, but can one ask why these elements are
different from the elements provided with the installation of Celestia?

Just curious...

Thanks, Brain-Dead

[selden]

CAP-Team,

What are you trying to say?
That that's a good value or bad value?

Which version of Celestia did you use?
Celestia v1.5.0 has had many improvements in the way it calculates trajectories.

The most accurate trajectories will be seen if you configure Celestia to use JPL's DE405 or DE406 ephemerides. See the Celestia WikiBook for details.

[selden]

Bob,

Celestia's orbital elements for Toutatis date back to 2001 and are only Keplerian elliptical parameters. The orbit has changed significantly since then, due mostly to coming relatively near the Earth in 2004. The parameters that CAP-Team has posted seem to be the Keplerian values predicted by Horizons for about the time of the next flyby in 2008.

[CAP-Team]

Bob,

Celestia's orbital elements for Toutatis date back to 2001 and are only Keplerian elliptical parameters. The orbit has changed significantly since then, due mostly to coming relatively near the Earth in 2004. The parameters that CAP-Team has posted seem to be the Keplerian values predicted by Horizons for about the time of the next flyby in 2008.

That is correct
I looked up the ephemerides for Toutatis on the Horizons system, for the date 1 november 2008. I use Celestia 1.5.0.

[BobHegwood]

Okay,

Thanks to you BOTH for my never-ending enlightenment here.

[CAP-Team]

I think the orbital elements of most asteroids and comets can be updated with much more accurate numbers from NASA's Horizons system, but question is at what epoch do you generate them, since they're more accurate at a time you know it passes at a known distance from another body.

I.e. you know that a spacecraft or whatever passes asteroid x at a certain time at a certain distance. If you generate the ephemeris for that asteroid at that time, the ephemerides will be accurate.
But Celestia doesn't take into account any pertubations the asteroid might undergo into the future or past.

With Celestia I try to simulate the solar system as accurate as possible. For the planets I use JPL's Planetary Ephemeris DE406 for planet positions and sometime when I want to explore a certain happening I check the ephemerides for all bodies that are involved. Sometimes correcting these ephemerides greatly improve the simulation compared with the figures that are published on the net, compared to the ephemerides that come with Celestia.

And Bob, I'm not a scientist or anything, just an astronomy enthousiast who just happens to love fiddling with computers

[BobHegwood]

And Bob, I'm not a scientist or anything, just an astronomy enthousiast who just happens to love fiddling with computers

Well, I certainly appreciate your explanations here. I'm not even an
astronomy enthusiast. I'm just some Bozo who happens to love the
curiosities and mysteries of the Universe, and I'm simply fascinated by
the way in which we can explore these mysteries with this spectacular
program.

By the way, I have updated my SSC with your data, and I very
much like to watch Toutatis as its perturbations manifest
themselves over a long time period.

Again, many thanks, Brain-Dead

[Nastytang]

Sorry to kick this old thread, but with these orbital elements I get a closest approach to earth of Toutatis on 9 November 2008 at a distance of 7.5 million kilometers (= 0.05 AU).

Code: Select all

EllipticalOrbit {
  Epoch       2454771.5       # 2008 NOV 01 00:00UT
  Period            4.03269897523155
  SemiMajorAxis     2.53355616628404
  Eccentricity      0.628990346621832
  Inclination       0.44552268775922
  AscendingNode   124.532940115730
  ArgOfPericenter 278.5457039835090
  MeanAnomaly     358.756418826834
}


Sorry for the newbie Question But which SSC files do we changes this INFO in

[selden]

Celestia/data/asteroids.ssc contains the definition for Toutatis.

Rather than modifying that file with a text editor, you should create toutatis.ssc in your Extras folder containing this text:

Code: Select all

Modify "Toutatis" "Sol"
{
EllipticalOrbit {
  Epoch       2454771.5       # 2008 NOV 01 00:00UT
  Period            4.03269897523155
  SemiMajorAxis     2.53355616628404
  Eccentricity      0.628990346621832
  Inclination       0.44552268775922
  AscendingNode   124.532940115730
  ArgOfPericenter 278.5457039835090
  MeanAnomaly     358.756418826834
}
}